AS Mathematics Pure Core 2 - Sequences and Series Help!

The sum of the first three terms of a geometric progression is 14. If the first term is 2, and the common ratio is r, show that r2 + r - 6 = 0

I tried substituting a = 2 and n = 3 into the geometric formula for the sum of a series, such that 2(r3 - 1) / (r - 1) = 14, but I can't get it to equal anything near the equation given in the question, so I think I'm doing something wrong...

Re: AS Mathematics Pure Core 2 - Sequences and Series Help!

The sum of the first three terms of a geometric progression is 14. If the first term is 2, and the common ratio is r, show that r2 + r - 6 = 0

I tried substituting a = 2 and n = 3 into the geometric formula for the sum of a series, such that 2(r3 - 1) / (r - 1) = 14, but I can't get it to equal anything near the equation given in the question, so I think I'm doing something wrong...